Catalysts For Methane Combustion Research Articles

Reduction and catalytic behaviour of nanostructured Pd/gadolinia-doped ceria catalysts for methane combustion

In the present work, 1wt% and 5wt% Pd/GDC10 (Gd0.1Ce0.9O1.95) nanopowders were prepared by two different methods: (a) cation complexation (CC) and (b) incipient wetness impregnation (WI) of an aqueous Pd2+ solution onto GDC10 nanopowders. All samples were characterised by X-ray diffraction (XRD), thermal analysis (TG/DTA), specific surface area determination and high resolution transmission electron microscopy (HRTEM). In order to study the oxidation state of Pd in samples with and without reducing treatments, XANES experiments at the Pd L3-edge were carried out. In situ Ce L3-edge XANES experiments were performed under reducing conditions in order to investigate the reduction behaviour of these materials. The addition of Pd to the GDC10 nanopowders increased the reducibility of Ce in the mixed oxide. Finally, catalytic tests for CH4 combustion were performed on the Pd/GDC10 nanopowders. Catalysts with higher Pd loading (5wt%) exhibited the best performance for CH4 combustion. The influence of preparation method was evident for catalysts with 1wt% Pd, the cation complexation method resulting in more active catalysts than the wetness impregnation method.

Effect of Nitrogen-Containing Impurities on the Activity of Perovskitic Catalysts for the Catalytic Combustion of Methane

LaMnO(3), either pure or doped with 10 mol % Sr, has been prepared by flame pyrolysis in nanostructured form. Such catalysts have been tested for the catalytic flameless combustion of methane, achieving very high catalytic activity. The resistance toward poisoning by some model N-containing impurities has been checked in order to assess the possibility of operating the flameless catalytic combustion with biogas, possibly contaminated by S- or N-based compounds. This would be a significant improvement from the environmental point of view because the application of catalytic combustion to gas turbines would couple improved energy conversion efficiency and negligible noxious emissions, while the use of biogas would open the way to energy production from a renewable source by means of very efficient technologies. A different behavior has been observed for the two catalysts; namely, the undoped sample was more or less heavily poisoned, whereas the Sr-doped sample showed slightly increasing activity upon dosage of N-containing compounds. A possible reaction mechanism has been suggested, based on the initial oxidation of the organic backbone, with the formation of NO. The latter may adsorb more or less strongly depending on the availability of surface oxygen vacancies (i.e., depending on doping). Decomposition of NO may leave additional activated oxygen species on the surface, available for low-temperature methane oxidation and so improving the catalytic performance.

Sol–gel synthesis combined with solid–solid exchange method, a new alternative process to prepare improved Pd/SiO2–Al2O3 catalysts for methane combustion

Abstract Various PdO–support interactions could be obtained on the Pd/SiO 2 –Al 2 O 3 catalyst prepared by the method adopted in this work. Higher palladium dispersion, lower palladium reduction temperature and better catalytic activity in methane combustion are obtained on the Pd/SiO 2 –Al 2 O 3 catalyst when PdO–Si interaction is favoured than PdO–Al interaction. In addition, compared to the Pd/SiO 2 and the Pd/Al 2 O 3 catalysts, higher catalytic activity is obtained on the Pd/SiO 2 –Al 2 O 3 solids due to their moderate acidity strength.

Effects of Carrier on Supported Rare Earth Double Perovskite Oxide Catalysts for Catalytic Combustion of Methane

Supported rare earth double perovskite (La2MnNiO6) oxide catalysts were prepared by incipient wet impregnation method for methane catalytic combustion, and effects of the support (Al2O3, ZrO2 and MgO) were investigated. The loaded catalysts were characterized by means of XRD, TPR, SEM techniques and their catalytic activities were tested by complete methane oxidation. Depending on the different oxide carrier (e.g. Al2O3, MgO and ZrO2), catalysts have different catalytic properties, because the interaction of metal oxide-support will affect both redox property and dispersity of the active phase. The results on catalysts have shown that the dispersion on Al2O3 and MgO supports had an effect to enhance the catalytic performances of the catalysts that had been treated at 1100 oC. The activity of ZrO2 supported catalyst significantly decreases when calcination temperature at is 1100 oC. The dispersity of the active phase and the nature of the oxide carrier played an important role in the catalytic performance.

Perovskite-like catalysts for the catalytic flameless combustion of methane

Modified LaCoO3 and LaMnO3 were investigated as catalysts for low tvemperature flameless combustion of methane. Modifications were carried out by the substitution part of La for Sr2+ and Ce4+, by the addition of 0.5% of Pt or Pd and by the substitution with Ag, which have limited solubility in the perovskite structure and may exist as intraframework Ag+ and extraframework metallic silver. Catalysts were synthesized by flame pyrolysis, which lead to a significant increase of both surface area and thermal resistance in comparison with the catalysts prepared by traditional sol-gel method. Samples were mainly characterized by XRD, BET and TPR techniques. Catalytic activity for the flameless combustion of methane was investigated by means of bench scale continuous apparatus, equipped with a quadrupolar mass spectrometer. In addition the resistance of every catalysts against sulphur poisoning was tested by using tetrahydrothiophene (THT) as poisoning agent. In most cases modification of perovskites led to an activity improvement, which was much more evident in the case of silver substitution. All the FP-prepared catalysts showed full methane conversion below 600°C, with CO2 and H2O as the sole detected products. Sr-substitution and addition of noble metals increased resistance to sulphur poisoning, while silver was not effective from this point of view, its main advantage being a substantial increase of the initial activity, which lead to satisfactory performance even after poisoning.

Enhanced hydrothermal stability of high performance lean fuel combustion alumina-supported palladium catalyst modified by nickel

The hydrothermal stability of Pd-based methane combustion catalyst was investigated under lean-fuel conditions, such as ventilation air methane. For the catalyst, the active component Pd was impregnated on Ni-modified alumina supports which were prepared by a modified method. The method was coupled with the conventional co-precipitation and hydrothermal synthesis process. A series of Pd catalysts and supports prepared, which had the Ni/Al ratios of 1:4, 1:2 and 1:1, were characterized by BET, CO-chemisorption, XRD, TPR, SEM and TEM. The relationship between active sites and supports was studied using HRTEM. In addition, the performances for lean methane combustion of all catalysts were studied. Results showed that the catalyst on well crystallized NiAl2O4 spinel support exhibited the superior hydrothermal stability. Methane conversion remained 96% after 3200h at 873K. Such excellent catalytic performance has been validated to relate to the stabilizing effect of support and the least lattice mismatch between NiAl2O4 and Pd, which contributed to high Pd dispersion.

Surface oxygen activation on CeO<sub>2</sub> and its catalytic performances for oxidation reactions

The research and development of rare earth catalytic materials will greatly promote the high-efficiency use of abundant rare earth elements, such as La and Ce. Based on the excellent oxygen storage/release capacity of CeO2-based catalytic materials, they have been widely used in many oxidation processes. In this paper, the recent progress of theoretical research on CeO2-based materials was reviewed, including the oxygen vacancy formation/diffusion on CeO2, the promoter effect of the carrier CeO2 and the nature of high oxygen storage properties of cerium zirconium solid solution. Through the discussion of the effect of Ce 4f orbit, special electron structure and structural relaxation on the aspects above-mentioned, the reasons for high oxygen storage performance and high oxidation activity of CeO2-based catalytic materials have been revealed. Furthermore, the catalytic performances of CeO2-based catalysts for the catalytic combustion of methane and halogenated hydrocarbons and CO oxidation were reviewed, and the relationship between the redox properties of CeO2-based catalytic materials and their catalytic activity for these oxidations were discussed. Finally, the problems and development thinking for the CeO2-based catalysts are put forwarded and outlooked.

A facile approach for the preparation of biomorphic CuO–ZrO2 catalyst for catalytic combustion of methane

A series of novel biomorphic CuO–ZrO2 catalysts were prepared using a cotton bio-template and compared with conventional CuO–ZrO2 catalysts. The physical and chemical properties of the as-obtained catalysts were characterized by techniques including X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), H2-temperature programmed reduction (H2-TPR), and O2-temperature programmed desorption (O2-TPD). The catalytic combustion of methane was chosen as the probe reaction. The results suggested that the bio-template method prepared porous biomorphic CuO–ZrO2 catalysts consist of hollow microtubes. Comparing with conventional CuO–ZrO2 catalysts, biomorphic CuO–ZrO2 catalysts displayed better reducibility and oxygen mobility, stronger metal-oxides synergistic effect, appropriate particle size distribution, and lower activation energy. The crystalline state of zirconia transformed from a single crystallite phase of t-ZrO2 into a complex of m-ZrO2 and t-ZrO2 after introducing the bio-template. With proper CuO content (20mol%), the biomorphic CuO–ZrO2 catalyst displayed preponderant properties. The compensation of surface lattice oxygen from bulk lattice oxygen was more available at high reaction temperatures.

Promotional effect of CeO2 and Y2O3 on CuO/ZrO2 catalysts for methane combustion

Catalytic combustion of methane was conducted by using a Cu-based catalyst prepared by the plasma-assisted impregnation method. The properties of the catalysts were surveyed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (H2-TPR). The results showed that the activity of CuO/ZrO2 with the CeO2 and Y2O3 was obviously increased compared with the CuO/ZrO2 catalyst, which was examined in relation to the structure and surface characteristics and might be correlated with their surface oxygen species and redox properties. Among the investigated catalysts, the Ce-CuO/ZrO2 sample exhibited the highest activity for methane combustion.

Preparation and Characterization of Perovskite-Type Oxide Catalysts for Combustion of Methane

A series of rare earth perovskite-type oxides La09Sr0.1Co0.9Mn0.1O3La08Sr0.2Co0.8Mn0.2O3and La05Sr0.5Co0.5Mn0.5O3were prepared along the sol-gel method and characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (SEM), transmission electron microscopy techniques (TEM) and catalytic activity measurement for combustion of CH4. The results indicated that a perfect crystal structure of perovskite was formed and La3+and Co3+ions were partly replaced by Sr2+and Mn3+ions, respectively, and perovskite-type oxides were composed of nanocrystals with particle size of 20~50nm. The catalytic activity for the combustion of CH4 was evaluated and La05Sr0.5Co0.5Mn0.5O3exhibited best performance with the temperature of 50% and 90% conversion efficiency of 723K and 833K.

Palladium-supported catalysts in methane combustion: comparison of alumina and zirconia supports

Palladium catalysts supported on alumina and zirconia were prepared by the impregnation method and calcined at 600 and 1000 ºC. Catalysts were characterized by BET measurements, XRD, XPS, O2-TPD and tested in methane combustion through temperature programmed surface reaction. Alumina supported catalysts were slightly more active than zirconia supported catalysts, but after initial heat treatment at 1000 ºC, zirconia supported palladium catalyst showed better performance above 500 ºC A pattern between temperature interval stability of PdOx species and activity was observed, where better PdOx stability was associated with more active catalysts.

Methane combustion activity of Pd–PdOx–Pt/Al2O3 catalyst: The role of platinum promoter

The effect of Pt promoter on Pd–PdOx/Al2O3 catalyst was investigated to improve understanding of the role of promoter formation of the active phase of Pd–PdOx–Pt/Al2O3 methane combustion catalysts. The influence of Pt addition on the composition of the active catalyst surface was characterized by NH3 temperature programmed desorption procedure. Addition of promoter improved the low temperature CH4 combustion activity of the aged catalyst, though it caused a decrease in activity of the fresh catalyst. The changes in activity were related to changes in the ratio of the metal to metal oxide phases in the surface composition. Pt tends to create metal sites, whereas Pd prefers the formation of metal oxide sites. Catalyst preparation parameters affect the Pd/PdOx ratio of the non-promoted monometallic Pd catalyst, and the ratio needs to be known so that the proper amount of Pt promoter can be added.

Synthesis and properties of PdO/CeO2-Al2O3 catalysts for methane combustion

This study focuses on the loading of catalytic materials, e.g., palladium on the surface of supporting materials, with the aim to obtain catalysts with high activity for methane combustion. The catalyst PdO/CeO2-Al2O3 was prepared by impregnation under ultrasonic condition. The effect of different activation methods on the activity of catalysts for methane catalytic combustion was tested. The properties of reaction and adsorption of oxygen species on catalyst surface were characterized by H2-temperature programmed reduction (H2-TPR), and O2-temperature programmed desorption (O2-TPD). Furthermore, the sulfur tolerance and sulfur poisoning mode were investigated. The results indicate that the catalyst PdO/CeO2-Al2O3 activated with rapid activation shows higher activity for methane combustion and better sulfur tolerance. The result of sulfur content analysis shows that there is a large number of sulfur species on the catalyst’s surface after reactivation at high temperature. It proves that the activity of catalysts cannot be fully restored by hightemperature reactivation.

Structural Evolution of Plasma-Sputtered Core–Shell Nanoparticles for Catalytic Combustion of Methane

Fe@Pd, Fe@Pt, and Fe@Au core–shell nanoparticles supported by silicon carbide have been prepared by plasma sputtering deposition and employed as the catalyst for methane combustion. The core–shell catalysts exhibit higher activities than single metallic catalysts due to surface alloying effects. With the surface alloying of the core–shell nanoparticles, Pd–O and Pt–O bonds become weak because the increase of the electron cloud density around Pd and Pt atoms due to the electron transfer from surface Fe to Pd or Pt atoms. Therefore, the activities of Fe@Pd/SiC and Fe@Pt/SiC increase with the reaction time. The activity of Fe@Au/SiC keeps invariant in the reaction because the Fe@Au core–shell structure has high stability. Transmission electron microscopy and X-ray photoelectron spectroscopy results further confirm the structural evolution.

Performance of silicon-carbide foams as supports for Pd-based methane combustion catalysts

BACKGROUND: Cellular foam materials are a new type of catalyst support that provides improved mass and heat transport characteristics and similar pressure drops to other well-established structured supports such as monoliths. RESULTS: A Pd-based catalyst has been prepared using a moderate surface area (25 m2 g−1) β-SiC foam support without further washcoating. The stability of this catalyst has been tested for methane combustion at lean conditions, showing a small decrease of activity during the first 10 h followed by stable performance. Characterization of fresh and aged catalyst shows no significant changes. The influence of the most important reaction conditions, such as reactor loading (0.25–1 g), temperature (300–550 °C) and inlet methane concentration (833 and 1724 ppm), was studied in a fixed-bed reactor. The results were fitted to three kinetic models: Mars-van Krevelen; Langmuir-Hinselwood; power-law kinetics. CONCLUSIONS: The Pd/β-SiC foam catalyst, prepared without the previous addition of a washcoating has been demonstrated to be stable for the combustion of methane-air lean mixtures. A Mars-van Krevelen kinetic model provides the best fit to the results obtained. Copyright © 2011 Society of Chemical Industry

Synthesis of La-hexaaluminate catalyst for methane combustion by a reverse SDS microemulsion

La-hexaaluminate catalyst for methane catalytic combustion was synthesized by a reverse microemulsion. Pseudo-ternary phase diagrams of a quaternary microemulsion system of sodium dodecyl sulfate (SDS), n-pentanol, n-octane, and water (or Al(NO3)3 solution) were presented. The effects of alcohol chain length, cosurfactant-to-surfactant ratio, and salt concentration on the formation and stability of the microemulsion system were studied. The phenomenon that the conductivity changed with water supported the phase behavior of the microemulsion system. La(Mnx/Fex)Al12−xO19−δ catalysts, applied in methane combustion and with high-temperature stability, were synthesized within the stable areas of the phase diagram of the microemulsion system, when SDS was chosen as surfactant, n-pentanol as cosurfactant, and n-octane as oil phase. The physical properties and structure of the catalysts were characterized by BET method, transmission electron microscope (TEM), and X-ray diffraction (XRD). A micro-fixed-bed reactor was used to measure the catalytic activity of hexaaluminates in methane combustion. The results show that the reverse microemulsions can be used to produce discrete La-hexaaluminate nanoparticles that display excellent methane combustion activity owing to their high surface area and high thermal stability.

Effect of intermediate layer on the activity and adhesion stability of metal monolith supported LaMn-hexaaluminate catalyst for methane combustion

Al 2O 3 and La 2O 3 layers were coated respectively on a FeCrAl alloy foil by a dip-coating technique and used as the second support for the active LaMnAl 11O 19 hexaaluminate (HA) phase in a metallic monolithic catalyst. A sample without an intermediate layer was employed for comparison. The properties and performances of the catalyst were examined with X-ray diffraction (XRD), scanning electron microscopy (SEM), ultrasonic vibration and thermal shock techniques. Methane catalytic combustion was performed to evaluate the activity of the catalyst. The results showed that the activity and adhesion of the HA to the alloy foil could be improved with the introduction of the intermediate layer. Al 2O 3 provided a strong adhesion, while La 2O 3 weakened the interaction between the active component and alloy foil. For the activity, the catalysts made with the two different intermediate materials also showed difference.

Synergistic Combination of Plasma Sputtered Pd–Au Bimetallic Nanoparticles for Catalytic Methane Combustion

Pd–Au bimetallic nanoparticles supported by silicon carbide have been prepared by plasma sputtering deposition and employed as the catalyst for methane combustion. It is found that the bimetallic nanoparticles consist of tightly-coupled Pd and Au particles, which are neither Pd–Au alloyed nor core–shell structured. The catalytic activity increases with the Pd loading in the catalysts. When the temperature is higher than 520 °C, Pd catalysts have an obvious drop in the catalytic activity due to the decomposition of PdO. However, the introduction of Au can delay and weaken the drop. It is indicated that there exists a synergistic combination between Pd and Au: oxygen transfers from Pd to Au at a temperature lower than 520 °C and from Au to Pd at higher temperature. Transmission electron microscopy and X-ray photoelectron spectroscopy results further confirm the synergistic combination.

A novel solid-gas process to synthesize LaMnO 3 perovskite with high surface area and excellent activity for methane combustion

Abstract A novel solid-gas route to prepare LaMnO 3 perovskite catalysts for methane combustion has been developed. The method was carried out using a polyvinylpyrrolidone-metal complex as precursor via a solid-gas process to obtain the target materials. The structure and properties of the precursor and the catalysts were characterized by FT-IR, TG-DSC, XRD and N 2 adsorption-desorption techniques. The results indicate that the catalysts synthesized via the solid-gas process possess higher surface areas, better thermal resistance and catalytic activity as compared to those prepared with the conventional sol-gel citrate method.

The influence of ceria and other rare earth promoters on palladium-based methane combustion catalysts

In this work we investigate the effect of ceria and other rare earths (La, Pr, Tb) on Pd–PdO transformation, which is strongly related to the catalytic activity of Pd-based catalysts for the combustion of methane at high temperature. Also the behavior of Pd on ZrO 2 and on yttria-stabilized zirconia is considered for comparison. The catalysts are characterized by XRD and BET surface area measurements. Pd–PdO transformation is studied by means of temperature programmed oxidation measurements and thermogravimetric analysis. Temperature programmed combustion experiments have been carried out in order to compare catalytic activity of different catalysts. The results show that rare earth oxides promote the onset of Pd re-oxidation upon cooling, that for ceria doped sample is reflected in an improvement in the catalytic activity.